RADIOACTIVITY

577

To Electrometer ,~~

§ (11) a RAYS, (i.) Range, of a Rays.—The a rays produce intense ionisafcion along their path in a gas, and, in consequence, they rapidly lose their kinetic energy until their velocity is reduced below the value at which they can ionise. Bragg and Kleeman 1 showed that the ionisation due to a homogeneous pencil of a rays ends after the rays have traversed a certain distance in air, this distance being called the range of the a particle in air. The range of an a particle from a simple produet is a constant for a definite temperature and pressure of the gas traversed. It varies inversely as the pressure and directly as the absolute temperature, so that in specifying the range it is important to state the temperature and pressure as well as the nature of the gas. Different products emit a rays of different ranges, so that the range of the a particle is characteristic of the product from which it is emitted.

Geiger and Nuttall 2 employed the following method for determining the range, and it is applicable to every kind of radioactive matter provided the latter is nut gaseous. The active material is placed in the form of a thin film on a small metal disc D in the centre of a metal bulb whose internal diameter is about 8 om. The disc, which is insulated from the bulb, is connected to one pair of quadrants of an electrometer, and the metal bulb is connected to a potential sufficiently high to produce the saturation current. Through the insulated stopper also passes a tube by means of which the bulb can be exhausted. The method of procedure was to measure the saturation current at different pressures. The results obtained are shown in Fig. 4. For low pressures the ionisation is very nearly proportional to the pressure, but when the pressure has reached a value such that all the a particles are completely absorbed in the gas, the ionisation current reaches a maximum value which remains constant with further increase of pressure, except in the case when two products in equilibrium were examined, in which case there are two abrupt breaks in the curve. The pressures at which the breaks in the curves occur correspond to the maximum ranges of the a particles in the gas at those pressures. The knowledge of this pressure enables the range in air at atmospheric pressure to be deduced, since the range is inversely proportional to the pressure.

1 Bragg and Kleeman, Phil. Mag., 1905, X. 318. 1 Geiger and Nuttall, ibid., 1911, xxli. 013 ; 1912, xxiii. 430 ; xxiv. 647.

VOL. IV

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Geiger3 had previously found that tho following relation existed between the range R of the a particle and its velocity V—

R=aV3, ... . (0)

i.e. the range is proportional to the cube of the velocity. The results of Geiger and Nuttall showed that a definite relation existed also

between the range and the radioactive constant of the substance emitting the radiation. This is shown graphically in Fig. 5.

If the range of the a particle of any product is known the period of this product can be deduced from this relationship. Tims tho period of avorago life of uranium II should bo about 3 x 10° years, and that of ionium 3 x 10B

FIG. 5.

years, periods too long for direct determination. Similarly from the long range a particles emitted by radium C' and thorium C', the very short periods of these products can be deduced; these would be of tho order of 10-° and 10"10 seconds respectively.

The curves show that those products whoso average life is long emit a rays whoso range is small, and vice versa.

" Goigor, Roy. Soc. Proc., 1910, A, Ixxxili. 5

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